Discrete power control of components within a computer system

ABSTRACT

Systems and methods for discrete power control of components within a computer system are described herein. Some illustrative embodiments include a system that includes a subsystem with a plurality of components (configurable to operate at one or more power levels), a control register (coupled to the plurality of components) including a plurality of bits (each uniquely associated with a one of the plurality of components), and a power controller coupled to, and configurable to cause, the plurality of components to operate at the one or more power levels. The power controller asserts a signal transmitted to the subsystem, commanding the subsystem to transition to a first power level. A first of the plurality of components, associated with an asserted bit of the control register, operates at a second power level corresponding to a level of power consumption different from that of the first power level indicated by the power controller.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application claiming priority toEuropean Patent Office Application Serial No. EP 06291604.4, filed onOct. 11, 2006, and entitled “Hybrid Power Control for Dynamic Low PowerDomains,” which is hereby incorporated by reference.

BACKGROUND

The proliferation of battery-operated portable electronic devices hasresulted in an increased demand for improvements in the power efficiencyof such devices. Many such devices now incorporate digital componentsthat are capable of transitioning into and out of a variety of reducedpower consumption states. Some computer systems, for example, may becapable of transitioning to a standby mode, wherein the processor ishalted, but power is still provided to both the processor and the memorysubsystem. Other systems may implement a sleep state, wherein theprocessor and other system components are powered down, but memoryremains powered, either at full power or at a reduced power level. Stillother systems may implement a hibernate state wherein the contents ofthe memory subsystem are saved to disk and the entire system is powereddown. The saved contents of the memory subsystem may be used to restorethe state of the system when the system is brought out of a hibernationstate.

Many of the systems that are capable of multiple power statesincorporate power control logic, sometimes in the form of a powercontroller state machine. The granularity of the control provided bysuch logic, however is limited to powering entire subsystems up or down,or reducing the power consumption of such subsystems by gating off aclock signal or by operating the subsystems at a reduced voltage (if thecircuitry is designed for such low-voltage operation). Further, thetransitions between power states are controlled by the power controllogic, with no direct power control of individual subsystems availablefor access and control by software executing on a processor within thesystem.

SUMMARY

Systems and methods for discrete power control of components within acomputer system are described herein. Some illustrative embodimentsinclude a system that includes a subsystem including a plurality ofcomponents (each component configurable to operate at one or more of aplurality of power levels), a control register coupled to the pluralityof components and including a plurality of bits (each bit uniquelyassociated with a single component of the plurality of components), anda power controller coupled to the plurality of components andconfigurable to cause the plurality of components to operate at the oneor more of the plurality of power levels. The power controller asserts asignal transmitted to the subsystem, commanding the subsystem totransition to a first power level of the plurality of power levels. Afirst component of the plurality of components, associated with anasserted bit of the control register, operates at a second power levelof the plurality of power levels corresponding to a level of powerconsumption different from that of the first power level indicated bythe power controller.

Other illustrative embodiments include an integrated circuit thatincludes a processor core, a memory subsystem coupled to the processorcore and including a plurality of memories, a control register coupledto the processor and the memory subsystem (the control registerincluding a plurality of bits, each bit associated with one of theplurality of memories), and a power controller coupled to the processorand the memory subsystem (the power controller configurable to controlthe power state of at least part of the memory subsystem). The processorasserts a bit associated with a first memory of the plurality ofmemories, and the controller asserts a power control signal transmittedto the memory subsystem. The asserted bit causes at least part of thefirst memory to operate at a first power level, and the control signalasserted by the memory controller causes a second memory of theplurality of memories to operate at a second power level associated witha power consumption level different from that of the first power level.

Yet further illustrative embodiments include a method that includesconfiguring each of a subset of components of a subsystem to operate ata first power level, signaling (to at least the subset of components) atransition to a second power level in which the subsystem consumes powerat a rate different from that consumed when operating at the first powerlevel, and the subset of components operating at the first power level(the configuring of each subset overriding the signaling to transitionto the second power level).

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of illustrative embodiments of the invention,reference will now be made to the accompanying drawings in which:

FIG. 1 shows a system incorporating individual power control ofcomponents of a memory subsystem, constructed in accordance with atleast some illustrative embodiments;

FIG. 2 shows the decode logic of FIG. 1, constructed in accordance withat least some illustrative embodiments; and

FIG. 3 shows a method for controlling power consumption of a computersystem, in accordance with at least some illustrative embodiments.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following discussion and claims torefer to particular system components. This document does not intend todistinguish between components that differ in name but not function. Inthe following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including but not limited to . . . ” Also, the term“couple” or “couples” is intended to mean either an indirect or directelectrical connection. Thus, if a first device couples to a seconddevice, that connection may be through a direct electrical connection,or through an indirect electrical connection via other devices andconnections. Additionally, the term “system” refers to a collection oftwo or more hardware and/or software components and may be used to referto an electronic device, such as, for example an integrated circuit, aportion of an integrated circuit, a combination of integrated circuits,etc. Further, the term “software” includes any executable code capableof running on a processor, regardless of the media used to store thesoftware. Thus, code stored in non-volatile memory, and sometimesreferred to as “embedded firmware,” is included within the definition ofsoftware.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims, unlessotherwise specified. The discussion of any embodiment is meant only tobe illustrative of that embodiment, and not intended to intimate thatthe scope of the disclosure, including the claims, is limited to thatembodiment.

Because of the complexity of modern computer systems, including systemson a chip or SoCs, it is not unusual for some subsystems within a systemto be heavily utilized for a given time interval, while other subsystemsare less utilized (or not utilized at all) during the same timeinterval. These subsystems group individual components that togetheroperate as and define a functional block of the system (e.g., a memorysubsystem, a graphics subsystem, and a direct memory access (DMA)subsystem). A significant power savings may be realized if, during thetime intervals when portions or components of the subsystems are notbeing utilized, such subsystem portions or components are placed in astate that causes the subsystems to consume less power than when fullyactivated. FIG. 1 shows an integrated circuit (IC) 100 (e.g., an SoC)capable of placing at least some individual components of a subsystemwithin IC 100 (including components within a component) into a reducedpower consumption state, in accordance with at least some illustrativeembodiments. IC 100 includes processor core 110, which couples to bothpower controller 120 and memory control and status register (MCSR) 130.Power controller 120 and MCSR 130 each couples to decode logic 140.Decode logic 140 couples to memory subsystem 200, which includesmemory[0] (210), memory[1] (220) and memory[2] (230). Each memory 210,220 and 230 respectively includes P-Logic[0] (periphery logic 212),P-Logic[1] (222), and P-Logic[2] (232). Each memory further respectivelyincludes array[0] (214), array[1] (224) and array[2] (234). As will bedescribed below, power controller 120 permits control of powerconsumption at the subsystem level, while MCSR 130 permits control ofpower consumption at the subsystem component level.

Each of the components within each memory of the illustrative embodimentof FIG. 1 (i.e., the array and the periphery logic) are powered fromseparately controlled power sources or “rails,” each of which is enabledby a control signal generated by decode logic 140. These control signalsdetermine which component is powered on or off, and, in at least someembodiments, at which of several power levels a component is operatedwhen it is not off. For example, the periphery logic within each memoryin FIG. 1 may be selectively powered-up (full power) or powered-down (nopower), independent of the power level at which the array of the memoryis operated. The memory array within each memory may similarly beindependently powered up or down, but may also be operated at one of twopossible powered up, power consumption levels. At the first “full-power”level, the array is on and operating at full power. At the second“reduced-power” level, the array operates in a “retention” mode, inwhich the array is operated at a power consumption level lower than thepower consumption level of the array during normal operation. Byincorporating split power rails and individualized control of eachsubsystem component, power control of the system may be extended beyondthe course control provided by a power controller external to thesubsystem. A higher granularity power control of the system is achievedby integrating the control mechanisms into the architecture and designof the subsystems and the subsystem components.

Continuing to refer to FIG. 1, the lower “retention” level of powerconsumption of the array is achieved in at least some illustrativeembodiments by operating the array at a reduced voltage level (while inthe retention mode) that is just high enough for the individual memorycells to reliably retain the values stored within them. In otherillustrative embodiments that utilize dynamic memory cells, aself-refresh mode may be enabled, by which refresh logic, powered by thesame power rail as the array, provides the minimal periodic accessesnecessary to maintain data integrity of the data stored within eachmemory cell of the array. Other techniques for operating the memoryarray in a reduced-power retention mode will become apparent to those ofordinary skill in the art, and all such techniques are within the scopeof the present disclosure.

The control signals that determine the power levels of the components ofmemories[0-2] are generated by decode logic 140, and include: Array Onsignals 0 through 2 (AON[0-2]), which, when asserted, cause the array ofeach corresponding memory to operate at full power; Retention On signals0 through 2 (RETON[0-2]), which, when asserted, cause the array of eachcorresponding memory to be operated at a reduced power; and Periphery Onsignals 0 through 2 (PON[0-2]), which, when asserted, cause theperiphery logic to operate at full power. By combining these signals andthe resulting power levels of the corresponding memory components,several modes of operations are defined for the illustrative embodimentof FIG. 1, as shown in Table 1.

TABLE 1 Mode Array On Retention On Periphery On Active Asserted (1)De-Asserted (0) Asserted (1) Retention Asserted (1) Asserted (1)De-Asserted (0) Sleep De-Asserted (0) De-Asserted (0) De-Asserted (0)RTA/AOD Asserted (1) Asserted (1) Asserted (1)

Referring to Table 1, the Active mode of at least some illustrativeembodiments is defined by an assertion of the Array On and Periphery Onsignals and a de-assertion of the Retention On signal. When a memory(e.g., memory [1] (220) of FIG. 1) is in the Active mode, the peripherylogic and the array of the memory (e.g., P-Logic[1] (222) and Array[1](224)) are both operated at full power. In the Active mode, the memoryis operated and accessed normally. In the Retention mode, both the ArrayOn and Retention On signals are asserted, while the Periphery On signalis de-asserted. In the Retention mode, the periphery logic is poweredoff, and the array is operated at a reduced power level. Data within thearray cannot be accessed by other logic within IC 100 when the memory isin the Retention mode, but the data saved in the memory is retained andcan be accessed by placing the memory into the Active mode.

In the Sleep mode of at least some illustrative embodiments, the ArrayOn, Retention On and Periphery On are all de-asserted, and both thearray and the periphery logic of the memory are powered-down. All datawithin the memory is lost once the memory is placed into the Sleep mode.In both the Retention-Till-Access (RTA) and the Active-On-Demand (AOD)modes the Array On, Retention On and Periphery On signals are allasserted. In both modes, the periphery logic is powered on, but thearray is maintained in a retention mode until data is needed. When datais needed, the Retention On signal is de-asserted, transitioning thememory to an Active mode. The data can be accessed more quickly whentransitioning from either an RTA or an AOD mode to an Active mode, whencompared to transitioning from a Sleep mode to an Active mode. This isdue to the fact that the periphery logic is already powered on in boththe RTA and AOD modes, whereas when transitioning out of a Sleep mode atime delay is necessary, before accessing the array, in order to allowthe periphery logic to stabilize upon being powered on.

Although the control signals that define each mode in at least someillustrative embodiments are in the same state for both the RTA and AODmodes, each represents a distinct mode within the context of power levelof the rest of IC 100 of FIG. 1. When a memory is in the RTA mode, atleast some of the other memories within memory subsystem 200 are in theActive mode. By selectively placing one or more unused memories into theRTA mode, the unused memories can be configured to consume less powerwhen not needed, while still retaining data. When a memory is in the AODmode, at least some of the other memories are in the Sleep mode. Byplacing the other, unused memories in a Sleep mode while placing in anAOD mode a memory used to store data that will be needed shortly (butnot immediately), power can be conserved while still making needed dataquickly available to a processor core when it is needed (e.g., withouthaving to swap in a saved memory image from a non-volatile storagedevice, such as a hard disk, upon transitioning from the Sleep mode tothe Active mode).

In order to operate memories within memory subsystem 200 in the RTA andAOD modes, it is necessary to be able to individually control the powerlevels of each memory, and of individual components within each memory.In at least some illustrative embodiments, such individualized controlis achieved by assigning a bit within MCSR 130 to each component withineach memory, and then combining control signals representative of eachbit with control signals from power controller 120, as shown in theillustrative embodiment of FIG. 2. The bits of MCSR 130 are organized ingroups of three, each group associated with a memory ([0], [1] or [2])within memory subsystem 200 of FIG. 1. The value of each bit within eachgroup determines the state of a signal that controls the power level acomponent within the memory corresponding to the group. Thus, forexample, bits 3, 4 and 5 of MCSR 130 of FIG. 2 respectively control thestate of the Retention On[1] (RETON[1]) signal 153, Active On[1](AON[1]) signal 154, and Periphery On[1] (PON[1]) signal 155, eachcorresponding to memory[1] (220) of FIG. 1. The PON[1] signal controlsthe power level of P-Logic[1] of FIG. 1, while the RETON[1] and AON[1]signals, in combination, control the power level of Array[1].

Referring again to FIG. 2, each individual control signal correspondingto the bits of MCSR 130 is combined with a control signal generated bypower controller 120 using an OR gate, generating each of the threecontrol signals used to determine the mode of operation of each memory.OR gates 141, 144 and 147 combine Power Controller Retention On(PC-RETON) signal 122 with MCSR 130 signals 150 (bit 0), 153 (bit 3) and156 (bit 6). OR gate 141 generates the RETON[0] signal, OR gate 144generates the RETON[1] signal, and OR gate 147 generates the RETON[2]signal. Similarly, OR gates 142, 145 and 148 combine Power ControllerActive On (PC-AON) signal 124 with MCSR 130 signals 151 (bit 1), 154(bit 4) and 157 (bit 7) to generate the AON[0], AON[1] and AON[2]signals respectively. Likewise, OR gates 143, 146 and 149 combine PowerController Periphery On (PC-PON) signal 126 with MCSR 130 signals 152(bit 2), 155 (bit 5) and 158 (bit 8) to generate the PON[0], PON[1] andPON[2] signals respectively.

Because logical OR gates are used to combine the signals, MCSR 130provides the capability of overriding several of the modes of operation,as commanded by power controller 120, for individual memories withinmemory subsystem 200. For example, if power controller 120 commandsmemory subsystem 200 to enter a sleep mode (PC-RETON=PC-AON=PC-PON=0),software executing on processor core 110 of FIG. 1 can set bits withinMCSR 130 to command individual memories within memory subsystem 200 toenter the Active, Retention, RTA or AOD modes of operation, overridingthe command from power controller 120. Other logical gates andcombinations of signals used to produce other overrides of either MCSR130 over power controller 120, or power controller 120 over MCSR 130,will become apparent to those of ordinary skill in the art, and all suchlogical gates, logical combinations of signals and overrides are withinthe scope of the present disclosure.

As already noted, the bits within MCSR 130 are accessible andcontrollable by software executing on processor core 110 of FIG. 1.These bits enable software executing on processor core 110 to controlthe power consumption of IC 100 with a granularity not otherwise easilyachievable using only power controller 120. Further, in at least someillustrative embodiments, because software executing on processor core110 has access to a wide variety of status information related to theoverall state of IC 100, the state of processor core 110, the state ofindividual subsystems and components within and coupled to IC 100, andthe state of software executing on processor core 110, it is possiblefor software that controls the MCSR bits to assert and/or de-assert thebits based upon complex combinations of status and state information.Such combinations may be too complex to implement within powercontroller 120, or may require information not easily accessible bypower controller 120 (e.g., program execution and state informationreadily available to an executive control program executing on processorcore 110, such as an operating system).

For example, in at least some illustrative embodiments multiple softwareprograms may be executing concurrently on processor core 110. Since onlyone program can execute at a time on a single core, each program isallocated a period of time or “time slice” in which to execute. In atleast some illustrative embodiments, the allocation of a time slices isperformed by the operating system under which the programs execute. Uponreaching the end of a time slice (or upon relinquishing a time slice ifthe program has no tasks to perform), a transition between the executionof one program and the execution of another program takes place, whichis sometimes referred to as a context switch. During the context switch,the state of the system (e.g., processor core register values, interruptmask values, and pipeline contents) are saved into memory for laterretrieval when the current program next executes (i.e., the currentprogram's next time slice). After saving the context of the currentprogram that is being suspended, the context of the program that isabout to resume (previously saved after completion of a prior timeslice) is read from memory and the state of processor core 110 isrestored.

To optimize the utilization and power management of memory subsystem200, context information is saved in one memory, while program data issaved in another memory. This is to take advantage of the fact thatcontext information is only needed during a context switch and notduring the execution of a program during its time slice. In at least oneillustrative embodiment, for example, the program data for both thesuspending and resuming programs is maintained in memory[0] (210) ofFIG. 1, while context information for both programs is maintained inmemory[2] (230). Throughout the execution of both programs (includingthe context switch), power controller 120 commands an Active On mode ofoperation. Executive control software (e.g., an operating system)executing on the processor tracks the state of the suspending andresuming programs, and configures MCSR 130 such that the mode ofoperation of memory[2] (230) is overridden and set to operate in an RTAmode. Memory[2] (230) is subsequently maintained in reduced power levelin between context switches, but is fully powered on during a contextswitch, when new context information is saved, and when previously savedcontext information is read. When the context switch occurs, theretention bit associated with memory[2] (which is in the RTA mode) isde-asserted by the executive control software, and memory[2] istransitioned to an active mode, thus “waking up” memory[2] and makingthe retained data available for use.

Although MCSR 130 is shown as a single, integrated control and statusregister, in at least some illustrative embodiments two separateregisters may be used within MCSR 130, one for control and one forstatus, each mapped to a single address within the address space ofprocessor core 110. Both writes and reads appear to processor core 110as described above for an integrated control and status register, butthe each status bits reflect the actual power state of eachcorresponding component rather than the state of the correspondingcontrol bit. The state of a status bit, in a split or separate registerconfiguration, may not transition to match the corresponding control bituntil after a time delay, due to the time it takes to actuallytransition the power state of the corresponding component. By usingseparate status bits to indicate the actual power state of thecomponent, rather than the commanded power state, it is possible to usethe state of the status bit, or a transition of the state of the statusbit, to trigger interrupts at processor core 110. These interrupts maybe used, for example, to cause software executing on processor core 110to respond to the actual change in the power state.

In other illustrative embodiments, the status bits of a split MCSR 130may be combined with other status and control bits within a component orsubsystem to detect command actions that are inconsistent with a powerstate. Thus, for example, if an attempt is made to access data in amemory that is in an RTA mode, the control bits used to access the datafrom memory may be combined with the status bits to produce an interruptindicative of the attempted, erroneous access, causing execution of anexception handler on processor core 110. Other combinations of controland status signals and bits, and other responses to changes in suchcontrol and status signals and bits will become apparent to those ofordinary skill in the art, and all such bits, combinations, andresponses are within the scope of the present disclosure.

FIG. 3 shows a method 300 for controlling the power consumption level ofa subsystem, in accordance with at least some illustrative embodiments.One or more bits within a control register, each bit corresponding to asubsystem component, are asserted so as to configure the correspondingsubsystem component to operate at a first power level (block 302). Powercontrol logic (e.g., power controller 120 of FIG. 1) is operated so asto signal a transition to a second power level that is different fromthe first power level (block 304). The asserted bits cause the subsystemcomponents corresponding to the asserted bits to operate at the firstpower level (block 306), while the subsystem components notcorresponding to the asserted bits operate at the second power level(block 308), ending the method (block 310). The asserted bits thusoverride the power level, as commanded by the power control logic, ofthe components associated with the asserted bits.

The above disclosure is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. For example, although theembodiments shown and described include components incorporated into asingle integrated circuit (e.g., an SoC), other illustrative embodimentsinclude systems with discrete components (e.g., discrete processors,memories and power controllers) incorporated into separate integratedcircuits, as well as integrated circuits with some components combinedtogether on a one integrated circuit and other components incorporatedinto separate integrated circuits. Further, although only some memorytechnologies are described, the present disclosure is not limited to anyspecific memory technology, and all memory technologies capable ofoperating at a plurality of power levels are within the scope of thepresent disclosure. Also, though the illustrative embodiments of thepresent disclosure are directed to controlling a memory subsystem,embodiments that include other types of subsystems with components thatcan be operated at multiple power levels may be used to achieve thepower consumption control described, and all such embodiments are withinthe scope of the present disclosure. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

1. A system, comprising: a subsystem comprising a plurality ofcomponents, each component configurable to operate at one or more of aplurality of power levels; a control register coupled to the pluralityof components and comprising a plurality of bits, each bit uniquelyassociated with a single component of the plurality of components; and apower controller coupled to the plurality of components and configurableto cause the plurality of components to operate at the one or more ofthe plurality of power levels; wherein the power controller asserts asignal transmitted to the subsystem, commanding the subsystem totransition to a first power level of the plurality of power levels; andwherein a first component of the plurality of components, associatedwith an asserted bit of the control register, operates at a second powerlevel of the plurality of power levels corresponding to a level of powerconsumption different from that of the first power level indicated bythe power controller.
 2. The system of claim 1, wherein a secondcomponent of the plurality of components, associated with a non-assertedbit within the control register, operates at the first power level inresponse to the transition signaled by the power controller.
 3. Thesystem of claim 1, wherein the subsystem is a memory subsystem.
 4. Thesystem of claim 1, wherein the plurality of power levels comprises afull-power level, a reduced-power level, and a powered-down level. 5.The system of claim 1, wherein the first component comprises a memoryarray within a memory device.
 6. The system of claim 1, wherein thefirst component comprises periphery logic within a memory device.
 7. Thesystem of claim 1, wherein the first component comprises a memory array,a third component comprises periphery logic, and a third power level isassociated with the third component; and wherein the first, second andthird power levels are each controlled independently by individual bitswithin the control register.
 8. The system of claim 7, wherein theconfiguration of each of the first, second and third power levelsdetermines a mode of operation comprising at least one mode selectedfrom the group consisting of an Active mode, a Retention mode, a Sleepmode, a Retention-Till-Access mode, and an Active-On-Demand mode.
 9. Thesystem of claim 1, further comprising a processor, wherein softwareexecuting on the processor controls the state of the bits within thecontrol register.
 10. An integrated circuit (IC), comprising: aprocessor core; a memory subsystem coupled to the processor core andcomprising a plurality of memories; a control register coupled to theprocessor and the memory subsystem, the control register comprising aplurality of bits, each bit associated with one of the plurality ofmemories; and a power controller coupled to the processor and the memorysubsystem, the power controller configurable to control the power stateof at least part of the memory subsystem; wherein the processor assertsa bit associated with a first memory of the plurality of memories, andthe controller asserts a power control signal transmitted to the memorysubsystem; and wherein the asserted bit causes at least part of thefirst memory to operate at a first power level, and the control signalasserted by the memory controller causes a second memory of theplurality of memories to operate at a second power level associated witha power consumption level different from that of the first power level.11. The IC of claim 10, wherein software executing on the processor corecauses the processor to assert the bit associated with the first memory.12. The IC of claim 10, further comprising a plurality of processorcores, each processor core coupled to the control register, and eachprocessor core configurable to control the state of one or more bitswithin the control register.
 13. The IC of claim 10, wherein the firstand second power levels are each set to at least one power levelselected from the group consisting of a full-power level, areduced-power level, and a powered-down level.
 14. The IC of claim 10,wherein each memory of the plurality of memories comprises an array andperiphery logic, and wherein power provided to the array of each memoryis controlled independently of power provided to the periphery logic ofeach memory.
 15. The IC of claim 10, wherein the bit associated with thememory is further associated with the array.
 16. The IC of claim 10,wherein the bit associated with the memory is further associated withthe periphery logic.
 17. A method, comprising: configuring each of asubset of components of a subsystem to operate at a first power level;signaling, to at least the subset of components, a transition to asecond power level in which the subsystem consumes power at a ratedifferent from that consumed when operating at the first power level;and the subset of components operating at the first power level, and theconfiguring of each subset overriding the signaling to transition to thesecond power level.
 18. The method of claim 17, wherein the signalingcauses at least one component not included within the subset ofcomponents to operate at the second power level.
 19. The method of claim17, wherein the first and second power levels are each set to at leastone power level selected from the group consisting of a full-powerlevel, a reduced-power level, and a powered-down level.
 20. The methodof claim 17, wherein the configuring of each of the subset of componentscomprises asserting bits within a control register, each bit uniquelyassociated with a single component of the subset of components.